A wide range of subcutaneous drug delivery devices are known in which a drug is stored in an expandable-contractible reservoir. In such devices, the drug is delivered from the reservoir by forcing the reservoir to contract. (The term "subcutaneous" as used herein includes subcutaneous, intradermal and intravenous.)
Such devices can be filled in the factory or can be filled by the pharmacist, physician or patient immediately prior to use. In the former case it may be difficult to provide the required drug stability in the device since the drug will be stored in the reservoir for a shelf life of from several months to a number of years. In the latter case, it is difficult to ensure that the drug has completely filed the reservoir, i.e. that the reservoir and fluid path do not contain any air bubbles. In general, this requires priming the device by filling it in a certain orientation which ensures that the air bubbles are pushed ahead of the drug, such as with the filling inlet at the bottom and the delivery outlet at the top (to allow the bubbles of air to rise during filling).
A further problem associated with subcutaneous drug delivery devices is that in many cases gas generation is used to compress the reservoir. While it may be possible to ensure a constant or a controllably varying rate of gas generation (for example by passing a constant current through an electrolytic cell), this does not ensure a constant rate of drug delivery.
The amount of compression of the reservoir (and thus the rate of delivery of drug) depends on the amount by which the volume of the gas generation chamber expands. The behaviour of an ideal gas is governed by the equation PV=nRT, in which the volume of gas, V, is proportional to the number of moles of gas, n, and the temperature, T, and inversely proportional to the pressure, P.
An electrolytic cell working at constant current will generate a constant number of moles of gas per unit time. However, changes in the temperature of the gas and in the atmospheric pressure exerted on the gas will cause the volume to vary. Even if the temperature of the device remains constant, the fact that atmospheric pressure drops by approximately 3% for every increase in altitude of 300 m means that the delivery rate will vary substantially between a location at sea level and a higher altitude location (for example, Denver, Colo. is approximately 5 miles or 8 km above sea level, so atmospheric pressure will be approximately 15% lower on average than at sea level). Similarly, normal changes in atmospheric pressure due to the weather cause the delivery rate of this type of device to vary.
For devices which employ a needle to penetrate the skin there is a danger that after use the device may accidentally infect the patient or others if not properly disposed of. Our WO 95/13838 discloses an intradermal device of this type having a displaceable cover which is moved between a first position in which the needle is retracted before use and a second position in which the needle is exposed during use. Removal of the device from the skin causes the cover to return to the first position in which the needle is again retracted before disposal. However, this device does not include a locking mechanism in the assembly for locking the device prior to use to minimise accidental contact with the needle and/or accidental actuation of the device that may occur during shipping and/or storage.
When filling a drug delivery device, the conventional method is to use a syringe, which carries the risk of accidental injury. The present invention has as a further aim the improvement of safety when syringes are used. The present invention also aims to decrease the possibilities that the needle could become exposed by accident before or after use, for example, by a child playing with the device if not properly disposed of. Clearly given the risks associated with infectious diseases, particularly those carried by blood, any possibility of accidental infection must be minimised to the utmost and preferably eliminated entirely.
Our International Application No. PCT/IE 96/00059 discloses a medicament delivery device having a filling mechanism integral within the housing which receives a cylindrical cartridge (or "vial") sealed by a sliding stopper. When the cartridge is pushed into the filling mechanism, a hollow needle in the filling mechanism penetrates the stopper and establishes communication between the interior of the cartridge and the device's internal reservoir.
Continued movement of the cartridge into the filling mechanism causes the stopper to slide into the cartridge and act as a piston to pump the medicament from the cartridge into the reservoir. While this mechanism overcomes some of the disadvantages of using a syringe, it also makes the device bulkier.
Thus, there is a need to provide a subcutaneous drug delivery device having an improved filling mechanism which facilitates filling the device in an orientation-independent manner.
There is a further need to provide a filling system that is less bulky.
There is still a further need to provide a filling system that maintains the needles within the system in a recessed fashion so as to minimise the risk of injury associated with needles.
There is yet a further need to provide a device which operates at a substantially constant delivery rate independently of the ambient atmospheric pressure.
There is a further need to provide a drug delivery device in which the needle is retracted from the housing surface before and after use so as to minimise injury due to accidental contact with the needle.
There is yet a further need to provide a device having improved adhesion to the skin, i.e. for which there is less likelihood that the device will become detached during use.